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Food is converted into energy in our bodies. Proper nutrition gives us proper amount of energy to cope with all our tasks. But why do we need energy?

There is simple experiment that shows one reason why energy is needed by the human body.

Measure and write down your body temperature.

How does your reading compare with the readings obtained by other people?

How do the results compare with normal human body temperature?

Measure room temperature by placing a laboratory ther¬mometer where the bulb is surrounded by air. Keep your hands well away from the bulb of the thermometer. After about two minutes, write down what the room tempera¬ture is.

Which is warmer, the air in the room or you?

What is the difference between the two readings?

In which direction will heat energy be travelling, from the air to you, or from you to the air?

If a beaker of hot water is left in the room, heat energy passes from the hot water to the air. What happens to the temperature of the water in the beaker. Does it rise, fall or stay the same?

You might expect the same to happen to human body temperature. Instead, it stays the same even though heat energy is being lost all the time. What must be going on all the time, in your body, to stop the temperature from falling as heat energy is lost?

So you see that the energy is needed to make heat. This heat replaces the heat that is lost by the body. It helps to keep body temperature constant at 37 °C. Energy is needed for other things as well.

Energy is measured in kilojoules. One kilojoule is the amount of energy needed to raise the temperature of 239 grams of water by one degree centigrade.

Energy requirements for A 25-year-old man, weighing 65 kg (10 stone), have such energy requirements: he needs 6 kilojoules per minute to just to sit, 7 to stand, 13 to walk slowly, and 38 to walk upstairs. A pregnant woman needs some extra energy.

Now let’s turn to measuring the amount of energy in a piece of food. When you eat food, such as a potato, you take in some of the energy captured from the sun. This energy was captured by the potato plant when it was growing. In order to make sure that we take in enough energy we must, therefore, find out how much energy there is in the foods which we eat and develop healthy diet for us. The average amount of energy taken in by an adult person is 8600 kj/day from from plant foods and 5200 from animal foods.

For most foods this is not easy to measure the energy that they contain. It requires the use of very complex equipment and very accurate measurements. However, a rough answer can be obtained for a peanut by setting light to it. The heat it produces is then used to heat up some water. The rise in temperature will give some idea of the amount of energy in the peanut.